1. Field of the Invention
The present invention relates to a master cylinder of a heavy duty truck clutch that has one or more switches for indicating the position of a clutch pedal.
2. Background Art
Heavy duty trucks are frequently manufactured with manual transmissions. Manual transmissions typically require depressing a clutch pedal to disengage the transmission and permit the transmission to be shifted between different gear sets. Other heavy duty truck systems may be integrated with a clutch pedal operation. For example, cruise control operation and engine ignition procedures generally require verification that the clutch pedal is in a particular position.
A cruise control interrupt switch may be provided as a normally closed switch that is opened immediately upon beginning depression of the clutch pedal. If a cruise control system remains operative while the clutch is disengaged, it may result in the engine racing while the truck decelerates as a result of disengaging the clutch. For this reason, a cruise control interrupt switch is frequently provided in association with a clutch pedal.
An engine ignition switch may be provided as a normally open switch that is closed when the clutch pedal is substantially fully depressed. It is desirable to preclude engine starting by the ignition system when the clutch is engaged to prevent the vehicle from lurching forward when the engine starts. An engine start switch is generally provided on a clutch pedal assembly at the maximum travel position of the clutch pedal to prevent engine ignition unless the clutch pedal is fully depressed.
In most instances two switches are provided on a clutch pedal box or as an elongated extension to the master cylinder. In the prior art, both switches in either the clutch pedal box or master cylinder embodiments use mechanical contact switches that are actuated by movement of the clutch pedal or master cylinder connecting rod. Mechanical switches are supported by the clutch pedal box and are contacted by the clutch pedal as it is moved during normal operation. Mechanical switches are subject to wear over time or may be damaged in use.
Some clutch systems may have a hydraulic power assist in which a master cylinder is shifted to cause a hydraulic circuit to shift a slave cylinder. The slave cylinder positions the clutch release bearing with a yoke mechanism. The use of a cylindrical or tubular mechanical switch section as an extension to a master cylinder has been proposed. An extended link piston rod must be used with the mechanical switch extension. The extended piston rod is provided with mechanical switch contacts that are open and closed as the piston rod moves relative to the cylinder. The mechanical switch extension is designed to be interposed between a protective rod boot and the master cylinder body. One problem associated with this approach is that the entire master cylinder assembly including the extended piston rod may be as much as two inches longer than a master cylinder that is not provided with the mechanical switch contacts. An increased length master cylinder is more difficult to locate on a truck and requires more space for installation and operation. The mechanical switch elements are subject to wear and may become contaminated with dirt and debris over time.
Applicant's invention is directed to solving the above problems and others as summarized below.